Stronger negative priming effect and lower basal respiration rates in nutrient-poor as compared to nutrient-rich forestry-drained peatland.

RATIONALE: Land-use changes, e.g., forestry drainage, modify the characteristics of peatland soil and affect the peatland carbon (C) balance. Peat soil nutrient status, related mainly to the original peatland type, also has an impact on the C balance after drainage, as observed earlier at the ecosystem scale for two forestry-drained sites in Southern Finland. Here the aim was to compare the soil CO2 fluxes from the two sites, nutrient-poor and nutirent-rich forestry-drained peatlands, and study the effect of plant photosynthates on the decomposition of peat C. Therefore, the respiration rates and priming effect (PE) of peat soils with variable nutrient status were examined in the laboratory. METHODS: Half of the samples were labelled with 13 C-glucose to study the effect of fresh C addition on the soil decomposition. The 13 CO2 -samples were analysed with isotope ratio mass spectrometry. A two-pool mixing model was applied to separate the soil- and sugar-derived respirations and to determine the PE. RESULTS: The nutrient-rich peat soil respired generally more than the nutrient-poor peat. A negative PE was observed in both peat soils, suggesting that the addition of fresh C did not increase the soil decomposition, but on the contrary decreased it. The negative PE was significantly more pronounced in nutrient-poor peat soil than in the nutrient-rich peat treatments, suggesting that the higher nutrient availability suppresses the negative PE. CONCLUSIONS: These results imply that microbes prefer utilizing fresh C instead of old C in the short term and that the peat decomposition is suppressed in the presence of fresh C inputs from vegetation at forestry-drained peatlands. These effects are even stronger in peat soils with less nutrients available. Ecosystem scale and soil process models could be improved with the help of these results.

Refining the role of phenology in regulating gross ecosystem productivity across European peatlands.

The role of plant phenology as a regulator for gross ecosystem productivity (GEP) in peatlands is empirically not well constrained. This is because proxies to track vegetation development with daily coverage at the ecosystem scale have only recently become available and the lack of such data has hampered the disentangling of biotic and abiotic effects. This study aimed at unraveling the mechanisms that regulate the seasonal variation in GEP across a network of eight European peatlands. Therefore, we described phenology with canopy greenness derived from digital repeat photography and disentangled the effects of radiation, temperature and phenology on GEP with commonality analysis and structural equation modeling. The resulting relational network could not only delineate direct effects but also accounted for possible effect combinations such as interdependencies (mediation) and interactions (moderation). We found that peatland GEP was controlled by the same mechanisms across all sites: phenology constituted a key predictor for the seasonal variation in GEP and further acted as a distinct mediator for temperature and radiation effects on GEP. In particular, the effect of air temperature on GEP was fully mediated through phenology, implying that direct temperature effects representing the thermoregulation of photosynthesis were negligible. The tight coupling between temperature, phenology and GEP applied especially to high latitude and high altitude peatlands and during phenological transition phases. Our study highlights the importance of phenological effects when evaluating the future response of peatland GEP to climate change. Climate change will affect peatland GEP especially through changing temperature patterns during plant phenologically sensitive phases in high latitude and high altitude regions.